Opioid- and GABA(A)-receptors are co-expressed by neurons in rat brain

Pharmacological data suggest that opioids exert their excitatory action in brain indirectly, by inhibiting release of the inhibitory neurotransmitter GABA. However, it is also possible that single neuron may interact with both opioids and GABA. In the present study, we investigated whether neurons in rat midbrain and medulla express both opioid and GABA(A) receptors. Coronal sections through rat brain were double-stained using antibodies against the alpha 1 subunit of GABA(A) receptor that were combined with antibodies either against the cloned mu-opioid receptor (MOR1) or the cloned kappa-opioid receptor (KOR1). Neurons double-labeled for GABA(A) receptors and either MOR1 or KOR1 were found in many brain regions including inferior colliculus, mesencephalic trigeminal nuclei, pontine reticular nuclei and raphe interpositus nucleus. Neurons double-labeled for GABA(A) and MOR1 were observed less frequently than those labeled for GABA(A) and KOR1. Our findings provide anatomical evidence that GABAergic and opioidergic systems are closely linked and activity of the same neuron may be regulated directly by both GABA and opioids.